This invention relates generally to a comparator circuit and, more particularly, to a comparator circuit which draws no supply current unless a voltage of a predetermined magnitude appears at an appropriate one of its inputs.
It is well-known that automobile manufacturers are tending towards the use of a multiplexed wiring scheme as opposed to the more traditional discrete wiring arrangement. This not only reduces the amount of wiring and connections necessary, but also reduces the labor of troubleshooting and improves long term reliability.
In addition, the automobile industry recognizes the need for diagnostic; i.e., the ability of an onboard computer to sense when, for example, a lamp is burned out or shorted, which lamp is burned out, and immediately inform the driver of the vehicle. Therefore, the multiplexed wiring system must include circuits which are capable of sensing intelligence at each element (e.g. lamp) and distributing this intelligence.
It is well-known that comparator circuits may be utilized to sense data on a data bus. The comparator trips when the voltage on the data bus reaches a predetermined threshold. Typical comparator circuits include a differential pair of transistors, the first of which has a control electrode coupled to a voltage divider circuit which sets the threshold voltage. The second transistor has a control electrode coupled to the voltage to be sensed. Unfortunately, this known circuit always draws supply current into the divider and into the differential pair. This is unsuitable for automotive purposes since, such a circuit would load down the battery even when the ignition key is off.